A Miniaturized 878 MHz Slotted Meander Line Monopole Antenna for Ultra High Frequency Applications

Transcription

1 Progress In Electromagnetics Research Letters, Vol. 67, 33 38, 217 A Miniaturized 878 MHz Slotted Meander Line Monopole Antenna for Ultra High Frequency Applications Nabilah Ripin *, Ahmad A. Sulaiman, Nur E. A. Rashid, Mohamad F. Hussin, and Nor N. Ismail Abstract A slotted meander line printed monopole antenna for low frequency applications of 878 MHz is presented. The novelty of the design is obtained by evaluating numerous slots and meander line that produces a new structure of double E-shaped meander line antenna backed by a defected partial ground plane. The operating frequency of the conventional printed monopole antenna is greatly reduced by the presence of the slots and meander line which lead to the reduction of the antenna size. The size reduction of 7% compared to the conventional reference antenna is achieved in this study. The antenna has a simple structure and small antenna size of 46.8mm 74 mm or.137λ.217λ.the antenna has been fabricated on a low-cost FR4 substrate and measured to validate the simulation performances. Measured results display that the proposed antenna produces omnidirectional radiation pattern of impedance bandwidth of MHz and the maximum gain of 1.18 dbi. 1. INTRODUCTION Ultra High Frequency (UHF) bands are extensively used in countless applications including LTE7/GSM85/9 band [1], wireless M-Bus [2], and wireless body area network [3]. The wavelength of the UHF frequency is much larger than the antenna available space leading to complexity of small antenna design [4]. Antenna miniaturization is required particularly for low frequency band systems where the size of the antenna is very large at this operating band. Beforehand, wire antennas were used for low frequency band applications including UHF band. Nevertheless, there were a number of matters need to be taken care of such as difficulties in integration with microwave integrated circuit board, and complexity in manufacturing. Planar antennas are one of the solutions to overcome these issues however with a limitation in terms of bulky size. In this study, a miniaturized printed monopole antenna based on slotted meander line patch at 878 MHz is proposed for UHF band. A small square defected ground structure is introduced to improve the impedance matching of the antenna. The evolution of the miniaturization techniques using slots and meander line is demonstrated in Section 2. Section 3 displays simulated and measured results of the proposed antenna. The proposed design has a small antenna size of mm 2 or about.137λ.217λ. 2. ANTENNA DESIGN AND APPROACH The miniaturization of a printed monopole antenna based on slotted patch and meander line is presented in this section. The antenna structures as revealed in Fig. 1 were designed and fabricated on an FR-4 substrate with dielectric constant ε r =4.3, substrate height h =1.6 mm and loss tangent of.25. The antenna is fed by a 5 Ω microstrip line which partially backed by a defected ground plane. The small Received 15 December 216, Accepted 25 March 217, Scheduled 3 April 217 * Corresponding author: Nabilah Ripin (nabilahripin@yahoo.com). The authors are with the Applied Electromagnetic Research Group (AERG), Faculty of Electrical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor 445, Malaysia.

2 34 Ripin et al. W s L 1 W p W 2 W 1 W 3 L a L 2 a W a W 4 L s W d L d W b W c L f L g L a W f W d W e Figure 1. Configuration of the proposed antenna. Front view. Back view. Figure 2. Configuration of the radiator (meander line is enlarged for visibility). rectangular slot with width and length of 3.7 mm is realized on the back side of the antenna to improve the impedance matching of the design. The proposed antenna has smaller and compact size of only 46.8mm 74 mm (.137λ.217λ )whereλ is a free-space wavelength at 878 MHz. All antenna configurations use the same substrate and feed dimensions. They have been simulated using Computer Simulation Technology (CST) Microwave Studio (MWS) software. Slots and meander line are introduced to increase an effective length of the current path with the aim to miniaturize the antenna. The increment of current path increasing the effective capacitance and inductance of the design leading to a decrement in the resonant frequency of the antenna as derived from the Equation (1). 1 f r = 2π (1) LC The dimensions of the patch, slots, meander line and the ground plane as presented in Fig. 1 are organized in Table 1. Table 1. Proposed antenna parameters. Parameters W p W s L s W f L f W 1 W 2 Unit (mm) Parameters W 3 W 4 L 1 L 2 L g W d L d Unit (mm) Figure 2 illustrates the front view of the proposed antenna with enlarged radiating element. The widths and lengths of the radiator as shown in Fig. 1 and Fig. 2 are calculated based on the equation below a = L p.5 (2) 2 W 1 =.139L p (3) L a = a 5 (4) 6 W a = a +1 (5) 4.72 W b = a +1 (6) 2.28 W d = W e =.69L p (7)

3 Progress In Electromagnetics Research Letters, Vol. 67, (c) (d) (e) Figure 3. Evolution of the proposed antenna. Ant. 1. Ant. 2. (c) Ant. 3. (d) Ant. 4. (e) Ant. 5. The proposed antenna shown in Fig. 3 has undergone a few evolutions before it was miniaturized. Ant. 1 represents a conventional printed monopole antenna covering wideband frequency including 878 MHz. Slots on the right and left side of the radiating patch are introduced as represented by Ant. 2 to increase the length of the current path hence decrease the resonating frequency. Another slot is introduced at the upper and bottom side of the patch as demonstrated by Ant. 3. To further reduce the resonating frequency a thin line at the center of the radiator is replaced by the meander line (Ant. 4). A square slot or defected ground structure is introduced to the back side of Ant. 4 to improve the impedance matching oftheantenna. Ant.1,Ant.2,Ant.3,andAnt.4havethesameantennasizeof83.79 mm mm but with different resonant frequency. Due to the reduction in resonant frequency after the evolutions, the size of the antenna can be reduced to bring back the resonant frequency of the desired frequency. The antenna size is then reduced to 7% so that the ant (Ant. 5) works at 878 MHz. The size of the proposed antenna after has been reduced is 46.8mm 74 mm or.137λ.217λ. 3. RESULTS AND DISCUSSION The simulated reflection coefficients of all antenna structures in Fig. 3 are explained in Fig. 4. Ant. 1 (reference antenna) and Ant. 5 (proposed antenna) resonate at the same frequency but with different physical size and structure. From the graph, it can be seen that the proposed antenna produce better reflection coefficient with better impedance matching. Although the bandwidth of the proposed antenna is narrow, it is does not affect the applications since it can be used for narrowband UHF applications. -1 S 11 (dbi) Ant. 1 Ant. 2 Ant. 3 Ant. 4 Ant Figure 4. Simulated reflection coefficient for various antenna configurations. Figure 5. Simulated surface current distribution of the proposed antenna.

4 36 Ripin et al. Ant. 2 produce lower resonant frequency compared to Ant. 1. Ant. 3 also operates at resonant frequency lower than Ant. 2 and Ant. 1 due to the slots. The presence of the meander line greatly reduces the resonant frequency of Ant. 4 from 51 MHz to 44 MHz. The size of Ant. 4 is then reduced so that it can operate at desired frequency of 878 MHz. Figure 5 shows simulated surface current distribution of the proposed antenna. From the figure, the maximum current is dominantly concentrated on the meander line of the center of the radiator and on the feed line. The maximum current also noticed at the edge of the slots that explains the reason how slots and meander line increase the electrical length of the current path. As mentioned earlier, the input impedance of the antenna is improved by the presence of defected ground structure. The input impedance can be seen in Fig. 6 with Z in = j.35 Ω at 878 MHz. The antenna structures have been fabricated and measured to validate the simulated results. The fabrication is done using LPKF milling machine while the measurement is completed in the Anechoic Chamber using Vector Network Analyzer. The photographs of the fabricated antennas are shown in Fig. 7. The measured results are compared with the simulated ones in the same graph. In Fig. 8, the simulated reflection coefficient is denoted by dash black lines while the measured reflection coefficient is denoted by solid red line. The simulated and measured resonant frequency are 878 MHz and 874 MHz, 6 4 Z in 2 Resistive (real) Reactive (imaginary) Figure 6. Simulated input impedance of the proposed antenna. Figure 7. Photographs of the fabricated antennas. Ant. 1. Ant Simulated VSWR Measured VSWR S 11 (dbi) -2 VSWR Sim. S11 Meas. S Figure 8. Simulated and measured reflection coefficient of the proposed antenna. Figure 9. Simulated and measured VSWR of the proposed antenna.

5 Progress In Electromagnetics Research Letters, Vol. 67, respectively. Figure 9 demonstrates the simulated and measured Voltage standing Wave Ratio (VSWR) of the proposed antenna. The results are agreed well with each other with VSWR less than 1.5. The radiation characteristics of the antenna are displayed in Fig. 1. A good agreement is observed between the simulated and measured results. Figs. 1 and display the radiation polar plot of the miniaturized antenna in E-plane and H-plane, respectively. The performances of the simulated and measured proposed miniaturized antenna are compared in Table 2. Discrepancies between simulated and measured performances occur due to the manufacturing and fabrication tolerances. Meanwhile, the discrepancies in antenna gain might be due to the electrically small antenna and ground plane effects. However, the measured antenna gain is still acceptable to the target application Sim. E-plane Meas. E-plane 27 Sim. H-plane Meas. H-plane Figure 1. Polar plot of the proposed antenna. E-plane (phi = 9 ). H-plane (phi = ). Table 2. Performance of the simulated and measured proposed antenna. Performances/ Parameters Sim. Proposed Antenna Meas. Proposed Antenna Frequency (MHz) S 11 (db) Efficiency (%) Gain (dbi) VSWR Bandwidth (%) Table 3 concludes the performances of the proposed antenna with other reported miniaturized antennas. It is discovered that the impedance bandwidth and gain of the proposed antenna are higher compared to [4 8]. The proposed work consists of a simple structure of smaller size approaching electrically small antenna and easier to fabricate. The size of the antenna can be further decreased but with degradation in antenna gain and efficiency. In order to not degrading the antenna gain and efficiency below than the acceptable range, the size of the antenna cannot be further decreased. As a matter of fact, wide bandwidth is not essential for most UHF applications such as Forward Scatter Radar Network. Therefore there is no necessity to have a wide band antenna where the size is more important.

6 38 Ripin et al. Table 3. Performance comparison of the proposed antenna with reference antennas. Ref. [4] [5] [6] [7] [8] Size f r (GHz) Gain (dbi) Bandwidth (%) Proposed work.29λ.33λ.74λ.139λ.74λ.137λ.53λ.71λ.41λ.139λ.118λ.217λ CONCLUSIONS A miniaturized printed monopole antenna based on slots and meander line concept is presented for UHF band applications at resonant frequency of 878 MHz. The size reduction of 7% has been achieved using the aforementioned techniques. The proposed antenna offers omnidirectional radiation pattern of 1.18 dbi measured gain and 41% efficiency. It is observed that the proposed antenna produces higher antenna gain and impedance bandwidth than the reported antennas in literature. ACKNOWLEDGMENT This research was funded by Universiti Teknologi MARA under Holistic Success Module code No. 6- IRMI/DANA 5/3/ARAS (28/216). REFERENCES 1. Zheng, M., H. Wang, and Y. Hao, Internal hexa-band folded monopole/dipole/loop antenna with four resonances for mobile device, IEEE Trans. Antennas Propag., Vol. 6, No. 6, , Loutridis, A., M. J. Ammann, and M. John, Folded meander line antenna for wireless M-bus in the VHF and UHF bands, Electron. Lett., Vol. 51, No. 15, , Jul Chandran, A. R., G. A. Conway, and W. G. Scanlon, Compact slot-loaded patch antenna for 868 MHz wireless body area networks, Loughborough Antennas and Propagation Conference (LAPC), , Mar. 28, 4. Babar, A., L. Ukkonen, M. Soini, and L. Sydanheimo, Miniaturized 433 MHz antenna for card size wireless systems, IEEE Antennas and Propagation Society, AP-S International Symposium (Digest), 5 8, Wang, X.-Y., Y. Fu, H. Wang, Y. Gui, and G.-M. Yang, A compact 433 MHz antenna with enhanced performance by using multi-resonant meander line structure, Asia Pacific Microwave Conference, 1 3, Gaetano, D., L. Loizou, J. Buckley, C. O Mathuna, K. G. McCarthy, and B. O Flynn, Compact 433 MHz antenna for wireless smart system applications, Electron. Lett., Vol. 5, No. 8, , Mitra, D., B. Ghosh, A. Sarkhel, and S. R. B. Chaudhuri, A miniaturized ring slot antenna design with enhanced radiation characteristics, IEEE Trans. Antennas Propag., Vol. 64, No. 1, 3 35, Liu, W., Z. Zhang, and Z. Feng, ISM 433-MHz miniaturized antenna using the shielding box of mobile terminals, IEEE Antennas Wirel. Propag. Lett., Vol. 11, , 212.